[Shu, Jiangbo;Cao, Taihe;Zhang, Xingfang;Zhang, Zhaoli;Liu, Hai;Li, Zhenhua] Cent China Normal Univ, Natl Engn Res Ctr E Learning, Wuhan 430079, Hubei, Peoples R China.^[Li, Zhenhua] China West Normal Univ, Network Ctr, Nanchong 637009, Peoples R China.